For decades wood flour has been used in wood plastic composites (WPC) for decking, fencing, and other applications. These products use a thermoplastic matrix resin, typically high density polyethylene (HDPE) or polyvinylchloride (PVC) which melts at elevated temperature so it can take the product form, via extrusion or molding processes, and upon cooling solidifies. In recent years other plant biomass fillers have been used in this manner and efforts to commercialize the fillers and concentrates for the thermoplastic industry have been gaining momentum.
In contrast, efforts to use wood flour and other bio-mass fillers in thermoset chemistries has been unsuccessful at 100% loading replacement for mineral fillers. Unlike thermoplastics, many thermosets historically have used high loadings of mineral fillers. Thermoset matrix, rubber and elastomer systems polymerized by use of free radical or ionic initiators are typically either liquid at room temperature or can be melted to a flowable viscosity prior to onset of cure. Once initiation is sufficient to start the polymerization the reaction auto accelerates and the irreversible cross-linking occurs. Once solidified the material cannot be melted to a molten state. Literature references note issues with inhibition and retardation of the polymerization mechanisms of these chain polymerizations initiated by free radical and ionic species with lignin containing plant biomass fillers. The mechanism of this interference has previously not been understood.
It is ironic that plant biomass filler technology has taken a quicker foothold with thermoplastics because historically they did not typically make use of the high density mineral fillers. Use of a lightweight bio-mass filler is desirable for thermosets because with a filler density of only ˜1 g/cc, vs. ˜2.5 g/cc for mineral fillers, and loadings as high as 36% based on volume the resulting composite weight can be reduced by more than 30% with equivalent resin and reinforcement content. Because precursors for the bio-mass fillers are abundant, can be locally sourced, and are easily harvested and processed they are projected to be cost neutral to mineral fillers on a volume basis. These advantages make the compounds utilizing the fillers attractive to transportation and aerospace applications, among others, where significant savings in fuel consumption are possible. Further the bio-mass fillers offer a reduced carbon footprint relative to mining operations, particularly when the precursor is a by-product of other products extracted or derived from the crop feedstock.
There has been a general assumption that due to the mechanical structure of lignin that its presence was an advantage as a polymer filler. Many literature references to bio-mass fillers and fibers in polymers specify lingo-cellulosic feedstocks. Many references also discuss heat treating of bio-mass fillers to reduce the hydrophilic nature.
Past efforts to utilize bio-mass based fillers in thermosets have been limited to partial replacement of 10-30% of total filler content due to the polymerization inhibition by the fillers. When high loadings of these fillers are used the resulting matrices have lower mechanical properties, increased water absorption and lower glass transition temperatures than neat or mineral filled analogs.
With fillers containing low aromatic content, however, this surprisingly is not the case. In our research we have discovered that selection of the bio-mass feedstock and design of modifying processes can be used to avoid or eliminate the inhibition problem. In fact we have found that fillers ground from low lignin and low aromatic content bio-mass do not have the inhibiting affect. Our studies have demonstrated that treatments involving heat aggravate the inhibition effect of lignin-cellulosics and can create the issue even with low-lignin feedstocks. Fourier Transform Infra-red (FTIR), Elemental Analysis (EA) and Nuclear Magnetic Resonance (NMR) studies have indicated formation of aromatic species in these treated fillers that resemble lignin and its decomposition products.
Thus, in spite of ongoing efforts to use bio-mass based fillers in thermosets and decades of non-success, we have discovered that avoidance, elimination or control of aromatic species level in the filler is critical for use in thermoset composites.